Various methods have been used to track the rotational speed of anything from car wheels to turbine engines. There are various approaches for capturing these rotational speeds. One approach is to attach a small generator, to a shaft or other rotating device which puts out a voltage proportional to the speed the generator is turning. This approach is often not desirable as it involves an additional mechanical connection.
Another approach is a magnetic tachometer having a Hall-Effect device, which changes voltage when a magnetic field passing through the tachometer changes. The voltage output is used to trigger an electronic circuit. These devices depend on the pulse being of a certain size to trigger the circuit and any attendant noise on the signal wire to be small enough not to trigger the circuit.
Another approach is optical. In this case a photo transistor or photo diode senses reflected light and when the light increases or decreases, a change in current occurs in attached circuit. This current is translated into a voltage, which is captured as noted above.
In these approaches there are three common factors: a. A rotating or oscillating machine; b. Coupling method (mechanical, magnetic, optical); c. A Circuit for detecting a voltage, voltage change, or current.
The electronics related to these approaches are tasked with ignoring electronic noise and detecting a true signal. Various methods have been used to achieve these tasks, but common methods comprise filtering noise electronically, while ensuring the signal level would be great enough or of sufficiently different frequency not to be filtered or ignored.
Of these approaches, optical systems are often selected. Optical systems include a light source and a photo sensor and a detector. The photo sensor can be, for example, a phototransistor or a photodiode or a charge coupled device (CCD). Light is emitted from the light source onto the moving part. (“Moving” captures all types of movement, including rotations and oscillations). The photo sensor captures reflections. The detector detects a difference contrast (lightness or darkness) on the moving.
In general, optical approaches face the problem of having enough light illuminating something of sufficient contrast to provide a signal big enough above the ‘noise floor’ to be considered valid. More specifically, problems involved in the optical approach include sufficient light; a paint spot, marked tape, color patch, (or some other optically differentiating part of the surface that add a different reflectance to the illuminating light source, and as detected by the photo sensor) which didn't fall off, fade, become tarnished, dirty, or discolored; and an electronic circuit that was tolerant of possible changes over time, optical path changes/variations, and rotating speeds. General electronics filtering and technology can be used for these purposes.
One method of handling these signals, since they do not change markedly, is to run them through a Phase-Lock-Loop. This is an electronic circuit that ‘seeks’ to oscillate in phase and at the same frequency as an incoming signal; but if there are some skips or small variations in the incoming signals, it will keep the output frequency steady. Thus, it is noise tolerant. This can be used since the signal does not change frequency suddenly. A car, for instance, will not come to a halt without braking, nor instantly go to sixty miles per hour without accelerating to that speed. (The only times things stop suddenly is because they hit something and the output of the tachometer is not likely to be instance under such circumstances.)
Another signal processing method is to take the AC signal (i.e., the changing part of the signal as opposed to the average or DC signal) and compare its crests to another voltage and when the crest exceeded the comparing voltage, an output pulse would be generated by the electronics to the system monitoring the speed. However this works when the signal does not change amplitude appreciably. As the moving mechanical part speeds or slows, there is a tendency for the signal to get small, since the reflected light or magnetic field is passing quicker, but this is again a slow variation with speed. However, this requires that an operator turn a knob (variable resistor) to adjust the comparison voltage to the right level to get valid output pulses.